The present invention relates to an electroconductive coating composition comprising a phenolic resin and a copper metal powder dispersed in said resin, and more particularly to an electroconductive coating composition with particularly improved electric conductivity, improved adhesion to copper foil and improved solder dip resistance for construction of the jumper circuit and electromagnetic shield of a printed circuit board, to a printed circuit board fabricated using said electroconductive coating composition and further to a durable flexible printed circuit assembly with electromagnetic shield.
As printed circuit boards for mounting IC, MSI, LSI etc., copper laminated insulation boards have been widely employed.
For insuring an effective utilization of the printed circuit formed on such a copper laminated insulation board, a bypass jumper circuit is provided on the printed circuit.
This jumper circuit is formed by the screen printing method which comprises forming a resist layer over the entire non-connected circuit region between copper foil circuits of a printed circuit and, then, applying a conductive silver coating composition (hereinafter referred to as a silver paste) between the copper foil circuits to be connected, skipping over the resist layer region. However, the silver paste is expensive. Moreover, although less expensive conductive copper coating compositions (hereinafter referred to as copper pastes) have been proposed as substitutes for the silver paste, these copper pastes are not dependable enough for use as the electroconductive coating composition for the formation of a jumper circuit because the use of a thermosetting phenolic resin as a main binder therein provides poor adhesion to the copper foil surface.
On the printed circuit of a printed circuit board, a shield layer is formed with a ground pattern or a resist layer interposed therebetween but the same problem in respect of adhesion to the copper foil surface of the ground pattern exists when said conventional copper paste is used for the formation of the shield layer.
Therefore, the inventors of the present invention previously proposed an electroconductive coating composition which is not costly and insures improved adhesion to copper foil and improved solder dip resistance (Japanese Patent Application No. 167229/1988).
However, the cured film of this coating composition was found to be deficient in the adhesion to the unoxidized copper foil surface.
To overcome the above-mentioned drawback, the present invention is intended to provide an electroconductive coating composition having improved adhesion to both the oxidized copper and unoxidized copper foil surfaces and improved electric conductivity and a printed circuit board fabricated using the same coating composition.